The function of all vitamins in the human body is extensive and complex. Vitamins serve multiple roles and they work together to perform many functions, from reducing infection to aiding metabolism. Taking in insufficient amounts of vitamins can result in deficiencies, and taking in excessive amounts can result in toxicities; both can cause health problems. A well-rounded diet with a variety of foods is ideal for supplying the body with an appropriate amount of the vitamins it needs to function to the best of its ability.
Vitamins can be categorized in one of two ways--fat-soluble vitamins and water-soluble vitamins. Vitamins A, D, E and K are the fat-soluble vitamins. They require the presence of fat for their absorption and are stored in the liver. Water-soluble vitamins include vitamin C and the B-complex vitamins. The B vitamins include thiamine, niacin, riboflavin, B6, B12, folate, biotin and pantothenic acid. Water-soluble vitamins are lost through urination and are not stored in the body.
Vitamin A is important for eye health and supports a healthy immune system. Beta carotene is a precursor to vitamin A and acts as an antioxidant which protects the body from free radical damage. Vitamin D assists with the absorption of calcium. Vitamin E is also an antioxidant, protecting healthy cells much like beta carotene. Vitamin K is important for blood clotting. Vitamin C plays a role in the synthesis of neurotransmitters important in brain function and collagen, a structural component of the body. It is important for wound healing and acts as an antioxidant. Thiamine is important in the function of the nervous system, helps release energy from the food we eat and promotes normal appetite. Niacin has a similar function to thiamine and also promotes healthy skin. Riboflavin is also like niacin and thiamine, in addition to promoting healthy vision. Vitamin B6 is important for protein metabolism, fat utilization and red blood cell formation. Vitamin B12 plays a role in the synthesis of genetic material, red blood cell formation and helps maintain a healthy nervous system. Folate prevents spinal birth defects and lowers homocystein levels. Biotin works to release energy from carbohydrates and to synthesize fat. Pantothenic acid plays a role in energy production and hormone development.
In addition to their functions within the body, vitamins are important for the prevention of disease. The antioxidant action of beta carotene, vitamin E and vitamin C protects cells, reducing the risk of cancer. In addition, vitamins E and C work hand in hand. Once vitamin E serves its purpose as an antioxidant it becomes inactive, however, vitamin C has been found to regenerate vitamin E. Vitamin A reduces the risk of night blindness. Vitamin D's role in bone health helps prevent osteoporosis. Folate's action to lower homocystein levels links it to a reduced risk of cardiovascular disease.
While it is rare to see major deficiencies of fat-soluble vitamins due to their storage, mild deficiencies can occur with the lack of a balanced diet. Children deficient in vitamin D can develop a bone disease called rickets. Lack of vitamin A can cause intestinal infections. Vitamin K deficiency can cause excessive bleeding. Deficiencies in water soluble vitamins are more common because they are not stored. Lack of vitamin C can cause infection and reduction in wound healing. Thiamine and niacin deficiency can result in mental confusion and muscle weakness. Insufficient intake of riboflavin and vitamin B6 can result in dermatitis. Folate and vitamin B12 deficiency can cause anemia. While uncommon, insufficient intake of biotin and pantothenic acid causes fatigue, nausea and abdominal cramping.
High doses of vitamins can result in toxicity. In much the opposite way as with deficiency, toxicity of water-soluble vitamins is rare, but it sometimes occurs in fat-soluble vitamins due to their storage in the body. Among water-soluble vitamins, abdominal cramping and bloating can occur with high doses of vitamin C, abnormal liver function can be seen with high niacin intakes and too much folate can mask a vitamin B12 deficiency, often causing anemia. Over-consumption of vitamin A can result in mild symptoms such as nausea and blurred vision, or more severe problems such as growth retardation. Too much vitamin D can cause nausea, weight loss, growth retardation and kidney damage. Vitamin E toxicity is uncommon, however, symptoms such as nausea and disorders with the digestive tract can occur. There are no reported toxicity symptoms with vitamin K.